THE EARTH’S OLDEST ROCKS
(THE ARCHEAN ERA)
Looking into the depths of Grand Canyon from any point within the Bright Angel section, one is immediately impressed by the narrow V-shaped gorge cut in the black rocks at the bottom. This is popularly termed the Granite or Inner Gorge. Within its walls one is in another world, both scenically and geologically. Their steep, bare sides, whose surfaces are chaotic in the extreme, have a history—long and complex. The rocks of which they are formed—some of the oldest known today on the surface of the earth—partially tell the story of the first great era in geologic history.
Other rocks of this, the Archean age, are found in the Rockies, in the Adirondacks of New York, and to a very great extent in eastern Canada. In the last named place they contain valuable deposits of iron, nickel, cobalt, and copper. Rocks which probably also correspond in age occur in Scandinavia, Brazil, China, India, and central Africa.
At the Grand Canyon, although we are impressed by the depth of the dark Archean rocks, beneath the plateau surface approximately a mile, yet we marvel even more when we contemplate their great age and the important series of events whose history they partially record. Built up originally as great horizontal deposits of sand and mud, they were bent by mighty crustal movements until high mountains, probably comparable to the present Alps, were formed. Pressures from the northwest and southeast apparently folded them. The rocks themselves were greatly compressed and heated, with the result that complete recrystallization and the development of a banded structure were brought about. The present vertical attitude of these ancient beds, together with their dense crystalline character, is evidence of the great depth at which they were formed and of the extreme pressures to which they were subjected. In brief, the rocks that we see today in the Canyon bottom represent merely the roots of once lofty mountains, and the flat surface cut on these rocks is an old plain that resulted from the wearing down of high country in this region.
BLOCK DIAGRAMS OF EVENTS OF FIRST ERA
1 Sands and Muds accumulated to form rock strata.
2 The strata were folded into high mountains. Heat and pressure changed both structure and composition of the lower rocks during the folding.
3 Molten masses were forced up into cracks from below. Upon cooling they formed lavas on the surface and granites beneath.
4 During a tremendously long period of time the rivers and rains slowly widened the valleys and wore down the ancient mountains to a great plain near sea level.
As yet no definite traces of either plant or animal life have been found in rocks of the Archean age in Grand Canyon. Though various forms of life may have existed then, and may have been preserved in the original rocks, their record has since been entirely removed by those extreme pressures which altered even the composition and structure of the rocks themselves.
GRANITE IN GRAND CANYON
(THE ARCHEAN ERA)
Within the black, crystalline rocks of the Inner Gorge may be seen many large streaks, bands or irregular masses of a lighter color. From the Canyon rim these appear white, but from nearby they are usually pink. These light colored rocks are granites with a coarse crystalline texture.
Granites derive their name from their granular texture. They are formed by the slow cooling of molten masses that have been forced into older rocks from the earth’s interior. From a similar source are formed lavas and volcanic ash, but these flow out or are ejected on the surface of the earth where they cool so rapidly that no crystals form. Exceptionally fast cooling or chilling of molten masses, moreover, forms volcanic glass or obsidian. It is by the application of this same principle that crystal forming is prevented in the manufacture of common glass.
The large size of the crystals forming the granite that fills cracks and fissures of the Inner Gorge at Grand Canyon indicates the considerable depth at which it was formed and is further evidence of the great mountains that existed in this region during the first era in geologic history.
